Sialic acids protective function

Knowledge concerning structural features of sialic acids necessary for their anti-recognition effect in erythrocytes is still scanty. The sialic acid side-chain seems not to be involved in this function, because shortening by periodate-borohvdride treatment does not significantly influence the viability of the erythrocytes.142 It is considered that the carboxyl group of Neu plays the main role in the protective effect of sialic acid residues. 

The usual procedure to remove the protecting silyl group (tetrabutylammonium fluoride in tetrahydrofuran) has caused acetyl rearrangements rather than to give a sialic acid derivative with a free C-9 primary hydroxyl function. Compound 5c, in which the free hydroxyl appears to be the secondary one at C-7, has been obtained as the sole reaction product in a 85% jdeld, after chromatographic purification. The overall yield of 5c based on 3a has been 83% [30]. 

One population has an exclusive specificity for the native type III antigen and is thus dependent on the presence of its sialic acid residues while the other has a specificity for the incomplete type III core antigen. There is evidence to suggest that this latter specificity is due to determinants terminating in B- -galactopyranose ( ) and also that this determinant could be functional in the production of protective antibodies (15). 

This review summarizes the latest developments in the chemistry of sialic acid which involve modifications of the natural 5-acetamido function (NHAc) into a) N-acetylacetamido (NAca), b) azido (N3), c) N-trifiuoroacetamido (NHTFA) and d) N-trichloroethoxycarbonyl (NHTroc). Last section is dedicated to protecting groups that have been introduced at the N-position but fiieir potential as sialosyl donors has not emerged yet, e.g. tert-Butoxycarbonyl (Boc), Benzyloxycarbonyl (Z or CBz) and 9-Fluorenylmethoxycarbonyl (Fmoc). 

Pyranosides show reactivities that dijfer as a function of sugar. Among commonly protected pyranosides (i.e., perbenzylated), reactivity decreases in the order fucose > galactose > mannose > glucose > sialic acid. These differences in reactivity are not significant. Fucose is approximately four times... 

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